As I said, I personally think it's a bad idea unless she is partied with underdark creatures of her same alignment. But of course the DM is god for his game. But you have to remember feats, and levels, and deities, and all kinds of stuff. Honestly it would be far easier to make her choose a different character.

But if you want to test your creative side then by all means go ahead.

"Today, my boyfriend and I went to the beach. I though he was being really sweet by putting sunscreen on my back as I layed on my stomach. I got home later, and felt that my back was sore. Then I saw the giant penis on my back that been burnt in. FML"

Tell them you aren't playing a game of Vampire: The Masquerade, and have him choose again.

The only other thing I can think of is to make him use the NPC mob stats. Honestly there are a ton of possibilities for PCs so why do they feel the need to go outside of the provided spectrum? Also, depending on your other Players, her being at all vampire may cause havoc since they are inherently evil. Thus, the PCs and most NPCs would always want to kill her.

I say it is a very bad idea. Very bad indeed. Although I like your write-up. But srsly, having a vampire in an adventure group would be worthless even if she is "good." Mainly because she won't be able to function well in the sunlight, and the other party members can't function as well in the darkness of night. Unless the rest of the party is made of Drow.